Process for coating copper and copper alloy

ABSTRACT

Copper or copper alloy sheet or foil displaying excellent solderability and high resistance to tarnish is prepared by the provision over its surface of a coating containing a copper salt of an organophosphonic acid. To form the coating, the sheet or foil is immersed for a short time in an aqueous solution containing a phosphonic acid, rinsed and dried, the treatment being combined with or preceded by oxidation of the sheet surface.

BACKGROUND OF THE INVENTION

The present invention relates to copper and copper alloy sheet or foilof significantly improved properties imparted by a novel thin invisiblesurface coating, and to the method of manufacture of such sheet.

Significant advances in this art have been described in U.S. Pat. No.3,837,929 issued Sept. 24, 1974 and in related patents such as U.S. Pat.No. 3,677,828 issued July 18, 1972. These disclosures teach theovercoming of difficulties and deficiencies, which were encountered infollowing earlier teachings, through the provision of a thin glassysurface film of copper phosphate, formed by treatment of the copper orcopper alloy substrate with a solution of phosphoric acid or of analkali phosphate, preceded by or combined with oxidation of the metalsurface. The thus-coated metal sheet displayed a desired high resistanceto tarnish and other desired properties.

The attainment of these advantages appears to be based, at least inpart, on the formation of a glassy pore-free coating of copper phosphatecontaining polymeric chains including multiple units of phosphorus atomslinked through intervening oxygen atoms in the surface film, which has athickness of about 20 to 1,000 Angstrom units.

However, such coating has generally been found to effect a significantdecrease in solderability, as compared to the clean uncoated metalsheet, such decrease being an important drawback for some uses. Forexample, such decrease in solderability detracts appreciably from one ofthe important advantages intended to be provided in treated electricalconnector parts, that of the ready soldering to other circuitcomponents.

It is, therefore, a principal object of this invention to overcome suchdeficiencies by providing a process resulting in a coated copper orcopper alloy sheet, wherein the coated sheet displays high values ofsolderability, while other desirable properties such as substantialresistance to tarnish are also accomplished.

It is a further object to provide a coated copper or copper alloy sheetor foil, and a method of preparing such coated sheet, wherein thesolderability of the sheet is maintained at substantially the initialvalue of the cleaned sheet prior to the treatment.

It is a further object to provide a coated copper or copper alloy sheetor foil, and a method of preparing such coated sheet, wherein initialproperties of the sheet are maintained substantially undiminished or areenhanced.

A further object is to provide a process which will efficiently andinexpensively provide such thin surface coating over a copper or copperalloy sheet or foil.

Further objects and advantages of this invention will become apparenthereinafter.

SUMMARY OF THE INVENTION

In accordance with the present invention, the foregoing objects andadvantages are achieved by applying to a copper or copper alloy sheet orfoil a solution of an organophosphonic acid, or salt thereof, for 4 to20 or more seconds at room temperature up to about 100° C., draining offexcess solution, rinsing, and drying, the treatment being combined withor preceded by oxidation of the sheet surface.

The organophosphonic acid, structurally an organic substitution productof phosphoric acid, is preferably one in which one of the three hydroxylgroups of phosphoric acid has been replaced by a monovalent hydrocarbonradical, which may be substituted or unsubstituted and which may besaturated or unsaturated, as by including ethylenic or carbonyl bonds.Generally, such organophosphonic acids or salts thereof arecharacterized as having a hydroxyl group of phosphoric acid replaced bya hydrocarbon radical, a carbon atom of which is linked directly to thephosphorus atom of the acid. Such radicals may include additionalsubstituents which may display C to N or C to O linkages.

Thus, the organo substituent of the phosphonic acid may consist of analkyl group such as methyl, ethyl, propyl, or higher alkyl; an arylgroup such as phenyl, propyl, or higher alkyl; an aryl group such asphenyl, napthyl, or higher aryl; an alkaryl group such as benzyl, phenylethyl, or higher; an alkene group such as vinyl, propene, butene, orhigher group; an acyl group such as acetyl, propionyl, butyryl, orhigher; an alkoxy group such as oxymethyl, oxyethyl or higher; a keto-group such as acetonyl, methyl acetonyl or higher; or a member of any ofthe above groups which may also include a hydroxyl, amino, or carboxylsubstituent. Likewise, mixtures of two or more of the above categoriesof phosphonic acid are effective to produce the desired coated copper orcopper alloy sheet or foil.

The treatment may be effected with an aqueous solution containing a lowto moderate concentration of the phosphonic acid component orcomponents, preferably ranging from about 0.1 to about 30 volume percentfor liquid acids or corresponding weight percent limits for solidphosphonic acids, preferably in the range of about 0.1 to 40 percent byweight.

The treating solution also preferably includes a low to moderateconcentration, such as about 0.1 to about 15.0 percent by weight,preferably 0.2 to 5.0 percent by weight of oxidizing agent, such assodium or other alkali chromate or dichromate, or nitric acid (100percent) at a concentration of about 0.05 to about 10.0 volume percent,perferably about 0.05 to about 2.0 percent by volume HNO₃. Other knownoxidizing agents of similar activity may be used at a comparable diluteor moderate concentration effective for the purpose, but generally withavoidance of such vigorous oxidizing conditions as might causesubstantial decomposition of the phosphonic acid.

Furthermore, it may at times be convenient to apply an oxidation stepseparately, prior to the treatment with the phosphonic acid component orcomponents. Such procedure may be advisable, for example, in instanceswhere the treating solution shows signs of some instability, as bychange in color, when stored in solution in the presence of oxidizingagent.

DETAILED DESCRIPTION OF THE INVENTION

The present invention may be applied with the use of the above reagentsand treatments, with the inclusion, if desired, of some procedural stepsas described in the patents referred to above. For example, the copperor copper alloy sheet or foil may be surface roughened prior to theformation of the improved surface coating.

The following illustrative examples in accordance with the inventionprovide specific details of procedure furnishing the desiredadvantageous results.

The copper or copper alloy strips treated in the following examples wereselected from the following known compositions:

    ______________________________________                                        C.D.A. Alloy No.                                                                           Composition                                                      ______________________________________                                        110          99.90 percent Cu (min.), 0.04                                                 percent O.                                                       194          2.1-2.6 Fe, 0.05-0.2 Zn, 0.01-0.04 P,                                         balance Cu.                                                      195          1.3-1.7 Fe, 0.6-1.0 Co, 0.4-0.7 Sn,                                           0.08-0.12 P, balance Cu.                                         260          68.5-71.5 Cu, balance Zn.                                        ______________________________________                                    

The strips had a thickness of 0.25 to 6.0 mils (thousandths of an inch),a width of 0.5 to 0.75 inch and length of 1.5 to 2.0 inches, and were inthe annealed condition. The strips were cleaned and degreased byswabbing with reagent grade benzene or chlorinated hydrocarbon, beforetreatment in the following examples, which involved the use of theorganophosphonic acid, or salt thereof, listed below.

    __________________________________________________________________________    No.                                                                              Organophosphonic Acid   Structure                                          __________________________________________________________________________    1  Methylphosphonic Acid                                                                                  ##STR1##                                          2  Ethylphosphonic Acid                                                                                   ##STR2##                                          3  Acetylphosphonic Acid                                                                                  ##STR3##                                          4  Propionylphosphonic Acid                                                                               ##STR4##                                          5  Hydroxyethylidene Diphosphonic Acid ("DEQUEST" 2010)*                                                  ##STR5##                                          6  Ethylenediamine                                                               Tetra(Methylenephosphonic Acid)                                               ("DEQUEST" 2041)*                                                              ##STR6##                                                                  7  Hexamethylenediamine                                                          Tetra(methylenephosphonic Acid)                                               ("DEQUEST" 2051)*                                                              ##STR7##                                                                  __________________________________________________________________________     *Product of Monsanto Chemical Corp.                                      

It will be noted that in the above organophosphonic acids, one of thethree hydroxyl groups in phosphoric acid has been replaced by an organicradical having a carbon atom linked directly to at least one phosphorusatom. Further, some of the above acids display a linkage of carbon tonitrogen or a linkage of carbon to oxygen in the organo radical. Thereagents may be added in the form of salts, such as the alkali metal orammonium salt, to the treating solution, which is maintained acidic, sothat the treatment of the metal surface is effected with thecorresponding organophosphonic acid.

Reagents and conditions for the specific examples are listed in thefollowing Table I.

                                      TABLE I                                     __________________________________________________________________________    Specific Examples                                                             Organophosphonic                                                                              C.D.A. Oxidizer                                                                             Temp.                                                                             Time                                        Example                                                                            Acid                                                                              Concn. Alloy  Concn. (°C.)                                                                      (Secs.)                                     __________________________________________________________________________    1    No. 1                                                                             18%                                                                              (wt.)                                                                             110 A* 3% (wt.)                                                                             25°                                                                        15                                          2    2   18%                                                                              (wt.)                                                                             110 A  3% (wt.)                                                                             25°                                                                        15                                          3    3   18%                                                                              (wt.)                                                                             110 A  3% (wt.)                                                                             25°                                                                        15                                          4    4   18%                                                                              (wt.)                                                                             110 A  3% (wt.)                                                                             60°                                                                        10                                          5    1   1% (wt.)                                                                             110 B**                                                                              0.1%                                                                             (vol.)                                                                            60°                                                                        15                                          6    5   10%                                                                              (vol.)                                                                            110 A  3% (wt.)                                                                             25°                                                                        20                                          7    5   10%                                                                              (vol.)                                                                            110 A  0.3%                                                                             (wt.)                                                                             25°                                                                        15                                          8    7   5% (wt.)                                                                             110 A  0.3%                                                                             (wt.)                                                                             25°                                                                        15                                          9    6   5% (wt.)                                                                             110 A  0.3%                                                                             (wt.)                                                                             25°                                                                        15                                          10   5   1% (vol.)                                                                            110 B  0.1%                                                                             (vol.)                                                                            100°                                                                       15                                          11   5   1% (vol.)                                                                            194 B  0.1%                                                                             (vol.)                                                                            100°                                                                       15                                          12   5   1% (vol.)                                                                            195 B  0.1%                                                                             (vol.)                                                                            100°                                                                       15                                          13   5   1% (vol.)                                                                            260 B  0.1%                                                                             (vol.)                                                                            100°                                                                       15                                          14   6   1% (wt.)                                                                             110 B  0.5%                                                                             (vol.)                                                                            100°                                                                       15                                          15   6   1% (wt.)                                                                             194 B  0.5%                                                                             (vol.)                                                                            100°                                                                       15                                          16   6   1% (wt.)                                                                             195 B  0.5%                                                                             (vol.)                                                                            100°                                                                       15                                          17   7   1% (vol.)                                                                            110 B  3% (vol.)                                                                            100°                                                                       15                                          18   7   1% (vol.)                                                                            110 B  1% (vol.)                                                                            100°                                                                       15                                          __________________________________________________________________________     *A = Na.sub.2 Cr.sub.2 O.sub.7                                                **B =  HNO.sub.3                                                         

The treatments in the above specific examples were carried out bypreparing aqueous solutions of the indicated compositions andmaintaining at the stated temperature, and partially immersing cleanstrips of copper or copper alloy, about 0.5 inch in width, to a depth ofabout 1.5 inches for the stated time period. After immersion, each stripwas rinsed in cold water and dried.

The coated strips were tested for resistance to tarnish by holding eachsample in the vapor emerging from a freshly shaken bottle containing a10 to 20 percent by weight aqueous solution of ammonium sulfide for 10to 15 seconds, whereupon untreated or incompletely treated areasdisplayed mottled surface colors of metal sulfide much duller and darkerthan the uniform bright metallic color of the well-coated treated areas.

Each of the above 18 specific examples of yielded strips displayingperfect resistance to tarnish over the entire portion which had beenimmersed in the treating solution, as such portions after exposure tohydrogen sulfide vapor, retained the lustrous bright metallic color ofthe initial strip. This was in sharp contrast to the variegated murkyand dark color shades shown by the unimmersed strip portions.

Such resistance to tarnish was found to be retained even after lengthystorage, as for 500 hours, in a laboratory cabinet.

The surface film produced on the metal strips by the immersion treatmentis transparent and invisible to the naked eye, but its presence as acoating which is substantially free of pores is established through theimprovements effected thereby in a number of properties, as illustratedby the vastly increased resistance to tarnish and the restoration ofsolderability to a desired extent.

Solderability tests were carried out on treated strips resulting fromthe above specific examples, and yielded the highest test rating withoutexception, even after lengthy storage of the treated strips in alaboratory cabinet, as for 500 hours. The solderability rating wassimilar to that observed in testing freshly cleaned initial metal stripswhich had not been coated.

The test rating was determined in accordance with the "Solder Dip Test",described on pages 57-58 of the article "Selecting Copper Alloys forContact Resistance and Solderability" by S. H. Butt in MetalsEngineering Quarterly, Vol. 12 (August, 1972), pages 55-62. In thistest, the strip is dipped into an alcohol solution of "Alpha Allros" No.100 Flux, then immersed vertically in a bath of molten 60 Sn-40 Pbsolder at 230° C. at a rate of 0.5 inch per second, held immobile in thebath for five seconds, withdrawn, cooled, and examined. The highestrating, Class I, is for a solder layer which is bright, smooth, and ofuniform thickness, with no surface irregularities or gaps. Ratings ofless perfect solderability fall in stages into Class II to Class V, thelatter rating being applied to test samples displaying no surfaceadhesion to the solder.

The U.S. patents set forth in this application are intended to beincorporated by reference herein.

This invention may be embodied in other forms and modifications withoutdeparting from its spirit or essential characteristics. The presentembodiments are, therefore, to be considered as illustrative of theinvention, the scope of which is indicated by the appended claims,changes coming within the range of equivalency being intended to beembraced therein.

What is claimed is:
 1. A method for producing a thin substantiallyinvisible and pore-free film over a copper and copper alloy surface,said film providing increased resistance to tarnish and improvedsolderability to said surface, comprising:(a) providing a materialselected from the group consisting of copper and copper alloys; (b)applying to said material a solution containing an organophosphonic acidand a chromate free oxidizing agent for at least 4 seconds to form saidfilm as a surface coating of a compound of said organophosphonic acid;(c) rinsing said material; and (d) drying said material.
 2. A methodaccording to claim 1, wherein the said acid is a phosphonic acid havinga structure derived by the replacement of one of the three hydroxylgroups of phosphoric acid by a hydrocarbon radical having a C atomlinked directly to a P atom.
 3. A method according to claim 1, whereinthe said acid contains a monovalent hydrocarbon radical.
 4. A methodaccording to claim 1, wherein the said acid contains an alkyl group. 5.A method according to claim 1, wherein the said acid contains an acylgroup.
 6. A method according to claim 1, wherein the said acid containsa hydroxylalkyl group.
 7. A method according to claim 1, wherein theorganophosphonic acid solution is maintained at a temperature of about20° C. to about 100° C.
 8. A method according to claim 1, wherein thesaid solution is applied to said material for 4 to about 20 seconds. 9.A method according to claim 1, wherein the said acid is applied inaqueous solution at a concentration of about 0.1% to 40% by weight. 10.A method according to claim 1, wherein the said oxidizing agent isapplied in aqueous solution at a concentration of about 0.1% to 15% byweight.
 11. A method according to claim 1, wherein the said oxidizingagent is nitric acid.
 12. A method according to claim 1, wherein thesaid oxidizing agent is applied to said material prior to applying saidorganophosphonic acid.
 13. A method according to claim 1, wherein thesaid acid has the structure ##STR8##
 14. A method according to claim 1,wherein the said acid has the structure ##STR9##
 15. A method accordingto claim 1, wherein the said acid has the structure ##STR10##